The invention relates to vehicle braking systems, and in particular to pedal simulator, also known as a compliance unit, for use in advanced braking systems where actulation of a brake pedal does not directly actuate hydraulic brakes, but rather acts to generate a braking demand signal which is used as an input to an electronically controlled vehicle braking system. Such advanced braking systems, are sometimes termed brake-by-wire braking systems or electro-hydraulic braking systems. In such systems, during normal braking operations, it is known to use a sensor to measure the displacement of the brake pedal, with the sensor generating a signal indicative of the distance the brake pedal is moved by the driver of the vehicle. This displacement signal is sent to an electronic control unit as a driver braking demand signal. It also known, during normal braking operations, to measure the force with which the driver depresses the brake pedal as another indication of the driver""s braking demand. This force may be measured directly by force sensors. However, the brake pedal typically is connected to actuate a piston, as in a master cylinder of a hydraulic braking system. A measurement of the pressure generated by the movement of the piston in a fluid system can be correlated to the force exerted by the driver on the brake pedal actuating the piston. Thus, a pressure sensor may be provided to sense the pressure generated in a master cylinder actuated by the brake pedal, with the signal produced by the pressure sensor being sent to the electronic control unit as another indication of the driver""s braking demand.
In advanced braking systems of this type, the master cylinder is normally isolated from the vehicle""s brakes during braking. Instead, the electronic control unit, responding to the braking demand signals, controls the operation of various pumps and valves to apply pressurized fluid to the vehicle brakes to achieve the desired braking action. Typically, in event of a malfunction, the valves would be repositioned to admit pressurized fluid from the master cylinder to the vehicle brakes.
It is desirable to provide a braking system which has a xe2x80x9cpedal feelxe2x80x9d which is similar to what drivers are used to in conventional hydraulic vehicle braking systems. That is, when the driver steps on the brake pedal with a certain speed and force, the driver expects to feel the brake pedal to respond with a characteristic resistance and movement to which the driver has become accustomed in operating conventional hydraulic vehicle braking systems. If the brake pedal of an advanced vehicle braking system is attached to actuate a master cylinder, the fluid in the master cylinder must be provided with a path to leave the master cylinder in order for the brake pedal to move (as expected) when the driver steps on the brake pedal, Typically, the fluid in the master cylinder is diverted to a pedal simulator, that is, an accumulator which receives pressurized fluid from the pedal simulator to permit the pedal to move as expected when the driver steps on the brake pedal.
The pedal travel is simulated to be that of a conventional hydraulic brake system. Typically, the pedal simulator is formed as a hydraulic cylinder with a movable piston therein acting against a coiled metal compression spring. The pressurized fluid from the master cylinder enters the pedal simulator, pushing the piston against the spring, which exerts a resisting force according to the spring characteristic of the spring. A conventional coil spring has a relatively linear spring characteristic of compression versus force exerted on the spring. This linear characteristic is reflected in a relatively linear response characteristic for pedal travel versus force exerted on a brake pedal in a brake system in which such a conventional coil spring is used.
This invention relates to a pedal simulator utilizing springs to provide a rake pedal with a non-linear, progressively increasing response characteristic for pedal travel versus force exerted on the brake pedal similar to a conventional hydraulic braking system. In a preferred embodiment, the pedal simulator is integrated into the housing of an associated master cylinder. The non-linear, progressively increasing response characteristic is provided in one embodiment by a conical spring having a non-uniform helix angle. In second and third embodiments, the non-linear, progressively increasing response characteristic is provided by a plurality of cylindrical coil springs.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.